1. Field of the Invention
The present invention relates to a fail-safe device for a motor vehicle and, more particularly to improvements in a fail-safe device for a motor vehicle speed governor of the type having a control system to switch a supply of power to an actuator ON and OFF to control the traveling speed of a motor vehicle to keep a constant speed.
2. Description of the prior art
Heretofore, a microcomputer has been used to control an actuator which drives a throttle valve in an intake manifold of an engine for providing a constant traveling speed of a motor vehicle. One of such computerized prior art throttle valve actuators with a microcomputer is shown in FIG. 1, wherein the microcomputer 17 generates a series of pulse signals having a predetermined repetition rate as a watch dog signal. This watch dog signal is differentiated at a watch dog signal processor 4. The differentiated signal is then rectified to cut off the negative polarity pulses and reverse the polarity thereof at the watch dog signal processor 4. This reversed signal is integrated at a reset signal generator 5. An amount of the integrated signal is brought substantially to 0 level, if a refresh signal is included which makes the reversed signal "0" level instantaneously synchronized with the rise time of the watch dog signal.
The value of the integrated signal is compared with a predetermined level at the reset signal generator 5 and if it is at or below the predetermined level, the watch dog signal is considered to be generated normally at the microcomputer 17 and the system remains as it is. However, if the value of the integrated signal exceeds the predetermined level for lack of the refresh signal, the reset signal generator 5 generates a reset signal since the microcomputer 17 is considered to be out of order because the watch dog signal is not generated properly at the microcomputer 17. The operation of the microcomputer 17 is then halted due to the reset by the reset signal for the initialization thereof.
In accordance with the prior art fail-safe device for the motor vehicle speed governor there is a possibility of no resetting of the microcomputer 17 even if a reset signal is generated under such conditions as an output of the watch dog signal ceasing and, worst of all, the generated signal for controlling the throttle valve to open remains as is which causes unfavorable problems in the safety of motor vehicle traveling.
Another prior art computerized fail-safe device for a motor vehicle speed governor is shown in FIG. 2. In this type of fail-safe device, a D flip-flop circuit (hereinafter called D.F.F.) 7 is provided as a latch circuit in addition to the microcomputer 17. The power supply to the actuator which controls the opening of the throttle valve for constant speed traveling is controlled in an ON/OFF manner in accordance with the output status of the D.F.F. 7. In operation, the D.F.F. 7 is set by a command signal for instructing the commencement of the constant speed traveling and is reset by a cancel signal for instructing the end of the constant speed traveling. The command signal and the cancel signal have been fed to the D.F.F. 7 from a command signal input means 2 and a cancel signal input means 3 being provided separately from the microcomputer 17. The power supply to the actuator is brought to ON when the D.F.F. 7 is set and to OFF when the D.F.F. 7 is reset. More specifically, the motor vehicle speed governor is not under operation in a normal traveling condition and the D.F.F. 7 is in the reset position or a canceled status. Therefore, a Q terminal of the D.F.F. 7 remains at a low "L" level and both transistors 9 and 12 are OFF for disconnecting first and second solenoid valve coils 13a and 14a from a power source 11. Afterwards, upon generation of a command signal from the command signal input means 2 by the operation of a driver the D.F.F. 7 receives the command signal at a CL terminal (i.e., clock terminal) for setting and the Q terminal is set to the "H" level since a D terminal thereof has been pulled up to a high "H" level. This turns on transistor 9 and transistor 12. Hence, the first and the second solenoid valve coils 13a and 14a are connected to the power supply 11 by the conduction of the transistor 12.
On the other hand, the microcomputer 17 is brought into operation to control the traveling speed of the motor vehicle upon receipt of the command signal from the command signal input means 2 and controls the ON/OFF state of the transistors 15 and 16 in accordance with an input signal from a speed signal generating means 1. The first solenoid valve coil 13a is energized when the transistor 15 is turned ON and the second solenoid valve coil 14a is energized when the transistor 16 is turned ON. Thereby, constant speed travel of the motor vehicle is provided by adjusting the opening of the throttle valve in accordance with the ON/OFF status of the first and the second solenoid valves 13 and 14.
In FIG. 3, a schematic diagram of the prior art actuator and throttle valve illustrating the structure and the relationship thereof is shown. Numeral 22 denotes an actuator consisting of solenoid valves 13, 14 and a vacuum diaphragm actuator 23 for controlling the opening and the closing of a throttle valve 25 provided at an intake manifold 24 of an engine during constant speed travel. The vacuum diaphragm actuator 23 comprises a housing 23b which defines an air chamber 23a vented to an input pipe 13b of the solenoid valve 13 an output pipe of which is connected to a negative pressure source of the engine and also vented to an output pipe of the solenoid valve 14 an input pipe of which is open to the atmospheric pressure, a diaphragm member 23c, a rod 23d for linking the throttle valve 25 and the diaphragm member 23c, and a spring member 23e interposed between the diaphragm member 23c and the housing 23b for pushing the diaphragm member 23c from the air chamber 23a side.
The operational conditions will be given by the following chart.
______________________________________ Operational 1st solenoid 2nd solenoid throttle valve mode (13) (14) (25) ______________________________________ acceleration ON ON OPEN deceleration OFF OFF CLOSE hold OFF ON CONSTANT ______________________________________
A cancel signal with a high "H" level is generated at the cancel signal input means 3 upon operation of the driver and is fed to the D.F.F. 7 at an R terminal for resetting the D.F.F. 7 since an S terminal thereof is grounded to a low "L" level. With this resetting, the Q terminal is brought to a low "L" level and the system is recovered to the normal condition for disconnecting the first and second solenoid coils 13a and 14a from the power source 11. The microcomputer 17 cancels the setting of the constant speed travel upon receipt of the canceling signal for halting control for the constant speed travel and stands by for a next command.
In this type of prior art speed governor, there is a danger of locking up the output signal which renders transistors 15 and 16 ON in case microcomputer 17 is in a runaway mode due to external noises or is malfunctioning. Further, the transistor 12 may be turned off if a cancel signal is derived from the cancel signal input means 3 and the D.F.F. 7 is reset therewith. However there is a problem for the safety of motor vehicle travel in the prior art speed governor. That is if a command signal is generated at the command signal input means 3 and the D.F.F. 7 is brought to the set condition upon receipt thereof, the first and second solenoid valves 13, 14 are energized and remain at the energized ON condition.
Still further accidental shortages between collectors and emitters of the transistors 15 and 16 may cause the same type of problem as described above in the prior art system.
As for the prior art fail-safe device for a motor vehicle speed governor, since an output of the D.F.F. 7 has not been monitored by the microcomputer 17, the control of the microcomputer 17 may differ from the output of the D.F.F. 7., if the D.F.F. 7 is rendered to erroneous operation because of external noises. Thus, the duplicate system for fail-safe operation does not effectively operate and has resulted in a lack of safety of motor vehicle travel.
It is, therefore, an object of this invention to provide a fail-safe device for a motor vehicle speed governor that can assure safety in travel by turning OFF a power supply to an actuator when a reset signal is generated due to the ceasing of the watch dog signal.
It is another object of this invention to provide a fail-safe device for a motor vehicle speed governor that can keep a power supply from providing power to an actuator when the malfunction or the runaway of the constant speed travel system has occurred, or the malfunction of switching elements for driving the actuator has occurred.
It is still another object of this invention to provide a fail safe device for a motor vehicle speed governor that can assure the safety of the traveling of a motor vehicle by normalizing an output of a latch through monitoring the output thereof.